Heretofore, flowmeters have been known that are provided in a pipe for allowing the flowing of fluid such as pure water and liquid chemicals used in a semiconductor manufacturing apparatus or the like, and measure the flow rate of the fluid that flows through the pipe. As the flowmeters, various types are known. For example, differential pressure type flowmeters are known. The differential pressure type flowmeter is an apparatus that detects the pressure of fluid on the upstream side of an orifice provided on a flow channel and the pressure of fluid on the downstream side of the orifice, with a pair of pressure sensors, and measures the flow rate of the fluid by a finite difference (differential pressure) between the detected pressures (e.g., see the publication of Japanese Patent No. 5220642).